The longest bike and pedestrian bridge in Greater Boston would also be its most expensive.

The proposal to connect the mega-developments of Assembly Square and the Wynn Resorts casino on each side of the Mystic River took a step forward last week, when state officials unveiled preliminary designs for a 780-foot footbridge — at a steep price of almost $23 million.

Now comes the hard part: figuring out who pays for it.

The government agencies involved in the project don’t appear in a hurry to volunteer, and although the biggest private beneficiary, the Wynn casino, is making positive statements, it also is stopping short of commitment.

The bridge would also require a separate project of $10 million or so to build a walkway over the train tracks to the Assembly Square Orange Line stop, a key feature that made it attractive to planners in the first place.

The $30 million-plus estimate marks a significant escalation since 2009, when the last study priced a footbridge at up to $8 million, a figure that proved to be too high and led to it being tabled.

Then came the rapid buildout of Assembly Row on the Somerville side of the river and the Wynn casino on the Everett side.

Perhaps most central to the bridge’s revived fortunes was the opening of the Orange Line station in 2014. The bridge would effectively put the casino within a 10-minute walk of the T, in a location so clogged with traffic already that city and state officials are planning significant improvements to the roads leading to the gambling resort.

The state agency overseeing the project, the Department of Conservation and Recreation, would only say that it is “seeking multiple funding sources,’’ including from the private sector, and that the project won’t move forward without a clear funding source. It added that it would be difficult to win private or federal money without spending on the next step to flesh out the bridge’s design.

David Mohler, a planner with the state Transportation Department, said at a recent public meeting that his agency “cannot commit’’ to funding the bridge, citing numerous competing priorities. And at the MBTA, spokesman Joe Pesaturo said it is “too soon to discuss any possible commitments’’ for the $10 million connection to the Orange Line station.

That leaves only a couple of other obvious candidates: the Wynn casino or the real estate company behind the buildout of Assembly Row, Federal Realty Investment Trust.

Wynn has so far provided $250,000 for the initial planning and is “positively considering paying’’ for the next phase of design and permitting, said construction director Chris Gordon, which will probably run to several million dollars. After that, he said, the company could kick in for some or all of the construction.

“We like the access to the MBTA, to the Assembly Row station,’’ Gordon said. “It works for us. But it also works for the whole district — the Somerville side and the Everett side.’’

Wynn is already on the hook for other transportation improvements as part of its requirement to mitigate traffic created by the casino, including running a ferry up the Mystic, shuttle buses to nearby T stations, and payments to help fund Orange Line service.

Federal Realty did not return requests for comment.

Even at more than $30 million, the footbridge would be a worthy investment with or without private support, said Peter Furth, a Northeastern University civil engineering professor.

“When you talk about how much money it costs to build a transit station or a line, this idea is a winner,’’ he said.

The proposed crossing was pushed by advocates long before Wynn appeared on the scene: It would provide a crucial link to miles of bike paths on each side of the river and improve transit access for Everett, a rare Boston neighbor without a subway or commuter rail stop. This section of Greater Boston has been growing fast, with new apartment complexes sprouting up, and the opening of the casino next year could lead to other nearby properties being redeveloped.

Moreover, one of the locations the state is pitching to Amazon for its second headquarters includes a strip of land between the Orange and Green Lines in Somerville and Cambridge that could be more accessible with the footbridge.

Amber Christoffersen of the Mystic River Watershed Association advocacy group said she is confident the bridge will get built because two big private companies and at least three state agencies are invested in it: the transportation and conservation departments, and the Gaming Commission, which oversees the casino.

“They have an interest in not just having wasted design efforts,’’ she said.

Meanwhile, the casino is finding itself under close scrutiny again from the Gaming Commission, which is investigating sexual assault misconduct claims that led to the resignation of the company’s namesake, Steve Wynn.

But the commission said it still supports the bridge and is considering a grant application to help fund design work for the portion around Assembly Station.

The proposed footbridge is expensive compared to others around Boston, largely because it mostly travels over water, requiring barges and piers for construction. The 600-foot Frances Appleton Bridge over Storrow Drive to the Esplanade now under construction will cost $12.5 million, while a footbridge connecting Cambridge and Charlestown along the Charles River cost $9 million.

Other big footbridges elsewhere in the country have typically hovered around $15 million, although one in Seattle over a highway and running 1,600 feet is now up to $38 million, according to the Seattle Times.

The Mystic crossing has several design challenges, including elevating the deck to 37 feet in parts to allow boats to pass and then ramping down to swing underneath a commuter rail bridge.

Adam Vaccaro can be reached at adam.vaccaro@globe.com. Follow him on Twitter @adamtvaccaro.